funding for this program was provided by... around the world lie the ruins of once grand civilizations. in colorado and new mexico, native americans built thriving towns. in the rain forests of mesoamerica, the ancient maya created magnificent city-states. here three million people once lived. in the earliest cradle of civilization, ancient mesopotamian farmers once made these deserts bloom.

halfway around the world, in california, are clues to understanding the fall of mesopotamia, as farmers here struggle to overcome a threat to this fertile garden land. the ruins of ancient societies may hold keys to our own survival as, out of the past, archaeologists explore one of the greatest of mysteries -- the decline and fall of grand civilizations.

mission control: ignition... and liftoff. liftoff... keach: for more than five millennia, humankind has seemed to dominate earth, both creating and destroying grand civilizations. each of these human experiments has changed our planet. this high vantage point brings us a new and sobering view.

for the first time, we behold our world as finite, limited. on the darkened face of earth, the lights of cities record the expansion of our kind. just 50 years ago, two billion people lived on earth. today our global population has reached five billion. within the next generation, it will double once more. our exponential growth now threatens the very resources that sustain life. the abandoned ruins of ancient societies hold clues to our survival. but to learn from our past, we must discard a romantic image of these earlier and more simple societies. archaeologist william sanders. a commonly held notion among the public at large and also among some of my anthropological colleagues is that non-western peoples live in harmony with nature, that they have relatively stable environmental relationships.

what archaeology teaches us is that that's not true. keach: in the new world, human beings appeared for the first time more than 10,000 years before the birth of christ. over the millennia, grand city-states emerged in the tropics of mexico, guatemala and honduras. this is the realm of the ancient maya. they built magnificent cities and peopled them with the portraits of their kings. these are the ruins of copan in honduras. the mystery of the rise and fall of these maya kingdoms has fascinated archaeologists for centuries. it is a puzzle presented in 1839 by the words and etchings of explorers frederick catherwood and john stephens.

"architecture, sculpture and painting, "all the arts which embellish life "had flourished in this overgrown forest. "the city was desolate. "it lay before us like a shattered bark "in the midst of an ocean, "her masts gone, her crew perished, "and none to tell whence she came, "to whom she belonged, "how long on her voyage, or what caused her destruction." now, 150 years later, the puzzle first put forth by stephens and catherwood is being pieced together. archaeologists are rebuilding the ancient city of copan -- a thirty-acre maze of ball courts and plazas, monumental sculpture and colossal stairways. intricate and beautiful maya glyphs

have been deciphered to reveal the history of a grand dynasty that once ruled here. high atop pyramids, the kings of copan conducted rituals to invoke the protection of their royal ancestors. on altars, they sacrificed sacred animals, like these jaguars, to appease a world of spirits. one such altar has been discovered at copan. altar "q" is inscribed with portraits of kings, the royal lineage of the 16th ruler of copan. archaeologist bill fash. now the reason behind altar "q" is to show that the 16th and final ruler of the site derives his right to rule from the fact that he's a descendant of each and every one of these illustrious royal ancestors. he makes this point very clearly by showing the founder in the act of passing the baton of office to him as king.

keach: the copan dynasty endured for almost 400 years, recording their triumphs on elaborately carved altars and stelae. carved here, too, is a record of their downfall. this is altar "l." it's the last carved monument at the site, and, in fact, represents a metaphor for the political collapse of this city. on it we see an attempt on the part of the 17th would-be ruler to copy the great dynastic altar, altar "q," of the 16th ruler. but in order to get to the throne part, you have to go to the back side of the altar. in so doing, what you discover is that the top and these two sides of altar "l" were never even carved. and, in fact, on the back side where you'd expect to find the rest of the text saying "seated as king," you just have a blank panel. they never finished carving the text. so on that date on the 10th of february in a.d. 822, when the sculptor dropped his tools and walked away, that was effectively the end of the copan dynasty.

keach: the fall of the maya dynasties was not only dramatic but widespread. within 150 years, many of the grandest maya palaces lay abandoned. at palenque, copan, tikal -- across a landscape the size of england, all was in ruin. archaeologist william sanders. all over the lowlands about 800 a.d., in hundreds of maya sites, they stopped erecting dated monuments, and they stopped building palaces and temples. this was an extraordinary historical event. a population of three million people or more drops down to a few tens of thousands. and it looked as though this happened over a period of only a century or a century and a half, according to the accepted chronology.

keach: the apparent swiftness of the collapse suggests a disaster -- a plague, drought or warfare. could the maya have been destroyed by foreign invaders ? here in mexico city is a vivid example of what often happens when one society conquers another. the mexican national palace stands in the center of the city. in its courtyard, the murals of diego rivera depict the spanish conquest of the aztecs. with guns and cannon, horses and armor, the conquistadores of hernan cortez attacked the armies of the aztec emperor, moctezuma.

for a time, the aztec orders of the eagle and the jaguar repelled the assault. but the aztec's traditional indian adversaries joined forces with the spaniards. the alliance destroyed the aztec empire. the spanish built mexico city atop the ruins of the aztec capital. in 1978, excavations for electric lines first uncovered aztec sculptures, including this huge carving of the moon goddess. today archaeologists have revealed the capital of the ancient aztec empire beneath spanish streets. they have reconstructed a great temple that was rebuilt and enlarged many times by the ancient aztecs.

the ruins of the temple lie beneath the main spanish plaza, called the zocalo. archaeologist william sanders. sanders: i think the zocalo of mexico city is the most exciting place on earth. in front of me you can see the material remains of three successive civilizations -- the great temple of the aztecs, the cathedral, a symbol of catholic spain, and, in the distance, the torre latino, which is a symbol of modern industrial mexico. prior to the aztecs, we have evidence of two earlier civilizations in the same valley. so we have successive civilizations, one replacing the other, for 2,000 years. and this is the normal pattern that archaeologists and historians find when they study a region of the world. what changes are the elite levels of culture -- the political institutions of power and the religious ideology that validates that power. but there is also a continuity. the fact that we have a city here

through three successive civilizations is an example of that. and that continuity exists because the changes only occur at the top. the bottom level, the working class, persists and provides the labor and the goods to support the system. keach: with stones from their own temples, aztec laborers were forced to build the spanish cathedral. a new religion replaced the old. but the cathedral is also a symbol of continuity in mexico. many who kneel to pray here are direct descendants of the aztecs. for the commoners, the spanish conquest and the later mexican revolution brought merely an end to one form of government and the imposition of another. it's a pattern repeated around the world. this is the ancient roman forum.

in the 4th century a.d., the empire finally crumbled. yet today, as in mexico city, life continues amidst the ruins of rome's former glory. the collapse of maya civilization was quite different. warfare and conquest cannot explain the total abandonment of these once grand cities. even if warfare had taken many lives, the maya population should have revived within a few generations. what could have caused such a widespread disaster ? 2,000 miles from the maya heartland, a similar puzzle intrigues archaeologists in the american southwest. nestled into the cliffs at mesa verde in colorado

are the abandoned ruins of the anasazi, the ancient ones. at chaco canyon, in new mexico, the anasazi built great residential and ceremonial centers. sometime between the 11th and 13th centuries a.d., all of these communities were abandoned. in the southwestern corner of colorado, one group of anasazi flourished at a place called sand canyon. now overgrown with juniper and pinyon, sand canyon pueblo was once a thriving, densely populated town. here, a team of archaeologists is excavating an abandoned anasazi settlement. woman: the bubble in the center. about 500 people once lived here in more than 400 rooms. the rooms are grouped around circular chambers, called kivas, where the anasazi practiced rituals.

the rituals were secret, so the kivas were built below ground level and concealed beneath an earthen roof. heavy beams, like this one, supported the roof. curiously, many beams are charred. the kivas had been burned. archaeologist william lipe. i'm not sure what it means, but i can speculate that the burning of the kiva roofs was some type of closing down ritual for this village. we've seen that a few other places in the northern southwest at a time when there was a substantial movement of population out of an area. so i think that's a possibility -- that they burned, intentionally burned, the kivas when they left this area for good. keach: to find out when the site was abandoned, a charred roof beam is wrapped and sent to a laboratory.

the anasazi often moved their villages, but usually only for a short distance. lipe: ordinarily when the anasazi left a settlement, they took most of their material equipment with them. if they weren't going very far, they might even take the roof beams. sand canyon pueblo is really different. this is a little rectangular ceramic box found in the niche in the kiva, probably left right where it was ordinarily left when they abandoned that part of the site. valuable little things like this ordinarily wouldn't be left by people. keach: so the archaeological evidence suggests that the people of sand canyon deserted their pueblo to migrate a great distance. what could explain this abandonment ? clues can be found in the traditions still nourished by a few anasazi descendants. rena swentzell is a pueblo indian

from the tewa community in new mexico. [ woman speaking native language ] interpreter: in our pueblo, we still dance the corn dance. we dance to tell the corn mothers how we appreciate their gifts. we want to thank them for a good life, for giving us the breath of life. keach: until a.d. 500, the indians of this area were nomadic hunters and gatherers of plants. at that time, they began to cultivate corn and live in settled communities. they were dry farmers, dependent on spring rains to germinate seeds and summer downpours to nourish growth.

anasazi ritual reflected the importance of corn for survival. and in these arid soils, the growth of corn depended on rain. this sample of wood contains a record of ancient rainfall patterns. the patterns are decoded by archaeologist jeffrey dean, director of the tree ring laboratory at the university of arizona. thousands of samples of trees have been collected here, an amazing record of rainfall that extends back to the time of christ. the vertical bands in each sample are growth rings. tree growth is largely determined by moisture. thick rings are created during years of high rainfall, thin rings during dry years. by carefully measuring the thickness of the rings, dean recreates the pattern of rainfall

when the tree lived. by sampling thousands of trees from across the southwest, he maps the ancient climate. tree rings from the sand canyon area disclose that from about 1270 to 1274, people experienced normal or above normal rainfall patterns, shown here in green. but low rainfall, shown in red, began in 1275 and lasted for 14 difficult years. lipe: so i think what happened was that people got in trouble here in the late 1200s with a severe drought. some people tried to stick it out, didn't make it, perhaps suffered famines or other calamities. but probably the majority of the people bailed out and joined related pueblo peoples to the south. this was also a period of extremely high population density in that area -- lots of people per square mile.

and it came at the end of a long period of farming in that area that depleted the agricultural resource. those three things put together created a situation that perhaps was almost unique in the history of that southwestern corner of colorado. keach: the collapse of anasazi society was triggered by drought, an act of nature beyond their control. population growth and depletion of this fragile arid environment may also have been factors. we often think of people like the anasazi as living in harmony with nature. just how common is it that ancient societies overexploit their environment and threaten their own survival ? at the state university of new york at stoney brook, a team of archaeologists seeks answers. elizabeth stone and paul zimansky study the plans of an ancient near-eastern city

they recently excavated. woman: so this must be the back wall of the palace. that you can pick up on the photograph there. keach: the city is called mashkan shapir. it was one of scores of ancient cities that thrived 4,000 years ago in the deserts of what is now iraq. this is the realm of ancient mesopotamia. centered around grand palaces and temples, urban life emerged in an area that archaeologists call the first cradle of civilization. today the ruins of these once grand cities crumble in the dry desert earth. but how could civilization have emerged in such an arid environment in the first place ? and what might have caused its destruction ? data from mashkan shapir provide clues. stone: we found a very large palace structure, which was decorated with baked clay pieces,

showing the city's own god -- nergal, the god of death. zimansky: there was a large temple area, in which we found pieces of sculpture -- lifesize and somewhat smaller -- of animals, human beings, all of which were probably part of the temple furniture. keach: they also found fish hooks, weights to hold down nets and fish spears -- all evidence there was once water here. written tablets like these describe mashkan shapir as a major port. but the city was 20 miles from the tigris river and 30 miles from the euphrates. how could a major inland port or any city, for that matter, survive in the desert so far from water ? stone begins a search for the source of water. for a perspective not possible from the ground, she orders a digital photograph of the area from the french spot satellite.

with artificial color, she analyzes the image on the stony brook computers. stone: in this blow-up, you can see the perimeter of the site right here, the two harbors -- one here and one less clear over here, which were fed by two main canals, one here and one here. the whole picture is one of a great deal of water at the site. in this view, you see one large channel here, which must have fed the site, which is located here. tracing the channel upstream, we suddenly found that it had two sources -- one coming here and the other here. the presence of the two channels explains not only how mashkan shapir could have had boat trade with both the tigris and the euphrates systems, but also the large amount of water that was available there. keach: the water flowed through a network of canals, allowing irrigation -- the lifeblood of civilization in this desert.

archaeological study of the canals presented an intriguing puzzle. within several decades of their construction, many of the canals were abandoned and new ones built to take their place. why would the mesopotamians abandon these waterways after investing so much labor in them ? halfway around the world, in the central valley of california, is an important clue. as in ancient mesopotamia, the government here constructed an elaborate system of aqueducts and canals. water from the nearby mountains has turned the fertile, but arid san joaquin valley into one of the most productive agricultural regions on earth. man: at the turn of the century, this was largely a desert -- jack rabbits and rattlesnakes, a few pioneering souls

that began small farming operations out here. keach: for larry turnquist and thousands of other farmers here, the desert began to bloom when large amounts of water were made available for irrigation. for all of its bounty, however, irrigation carries the seeds of its own destruction. turnquist: this area, the san joaquin valley, has the ability to continue to be the bread basket for the world for many, many years to come. but in order to continue this abundant production here, we have to solve one enormous problem... salt. keach: salts like sodium chloride and calcium carbonate are turning some of the world's most productive farms into saline wastelands. in many of the world's arid regions, salt is a natural component of soil and water.

when irrigation water evaporates, salts are left behind, contaminating the soil. to prevent this, additional water must be applied to wash the salts down and away from the root zone. the great quantity of water cleanses the soil. but in the process, it, too, becomes saline. deep beneath these fields, layers of rock or clay form a barrier. with nowhere to go, the saltwater rises towards the roots, inhibiting growth or killing the crop. in wells throughout the valley, technicians easily measure the short distance to the saline water table, and test for the salt content. the saltwater has risen almost to the surface beneath nearly a million acres. experiments like this canal to take the saline water to the sea

have largely failed. turnquist: as we irrigate, we continually add a little salt to the soil. and i look at some of my neighbors' lands, and i can see the saltwater right at the surface. and i look at one parcel of my ranch, and the salt's getting closer and closer. and as the salt gets closer and closer, i see losing my ranch. keach: by the 18th century b.c., civilization had largely abandoned southern mesopotamia. evidence of salt damage here can be found in ancient texts. clay tablets like these describe a host of changes in the harvest due to salt in the fields. archaeologists continue to debate the causes of this collapse, but one fact is clear --

irrigation and salinization contributed to the decline of city after city. in this cradle of civilization, humans demonstrated they could destroy their environment as quickly as they could master it. but there are no deserts in mesoamerica. what can explain the maya collapse ? the majority of their cities flourished in a broad tropical rain forest, one of the wettest environments on earth. here irrigation was insignificant, and there was no evidence of ancient climatic change. a drought simply cannot explain the maya downfall. and there is no evidence of other natural disasters that might have caused such a massive decline in population. for more than 150 years, scholars have studied the maya.

gracias. sí. la otra. keach: but for most of this time, they focused on the royal centers. bien. at copan, they have discovered the burials of kings and their retainers. we now know the names of each king in a long royal dynasty and the date, almost to the day, when the dynasty ended. over the years, the grand temples and palaces of the acropolis have been reconstructed. but for all this impressive research, one essential question long remained unanswered. who were the common men and women who lived here ? when and how did they disappear ? this looks great. now, this area is so intensively used... to find out, bill sanders and dave webster,

archaeologists from penn state university, began a project in 1980 that would span a decade of work. sanders: before we srted the copan project, we knew a great deal about copan. and its history was very similar to that of many classic maya sites. there was a long period of population growth, of political evolution, peaking around 800 a.d., and then the dynastic sequence ends shortly afterwards. and within a short period, the valley seems to have been abandoned. what we didn't know was anything about the factors and the mechanisms and the processes that produced this peculiar population history. and what we decided to do was a series of excavations and surveys which would allow us to find out how big the population was, how it was distributed on the landscape, how those people made a living off of this landscape, what the various social classes and social groupings were, and finally, hopefully, to get enough chronological information to find out how long this whole process of fluorescence and decline required.

keach: their goal was nothing less than the recreation of the entire society of ancient copan. to understand what life had been like for the vast majority of the maya, research teams fanned out throughout the valley. here, far from the palaces of the kings of copan, archaeologists discovered the ruins of thousands of buildings -- the homes of the common farmers. the way ancient settlements are patterned on the landscape is an archaeological clue to how the maya may have organized themselves and how they might have exploited their environment. man: poquito más arriba, por favor. to decipher the pattern, the team would explore and map the entire valley -- a colossal task of surveying more than 60 square miles. poquito más arriba, por favor. today, all that remains of maya dwellings are subtle changes in elevation

barely detectable to even a trained eye. yeah. there's a corner. mira, arturo. a corner. this was once the home of a rural maya peasant. webster: perfecto. es así. 3 meters and 80 centimeters. another good-sized mound. and here's where we came in. together with surveys completed by earlier expeditions the team mapped more than 4,500 house mounds. the map shows the extent of maya population in the hillsides surrounding the royal center, but it tells us nothing about when the population rose and fell. the sites would have to be dated. a sample of the house mounds is excavated to determine when people lived here and for how long. ah, that's what we need. a nice piece of obsidian.

to date the hundreds of sites they excavate, the team would try an ingenious dating technique. the key to the technique is obsidian. razor-sharp obsidian knives were found at almost every site. each obsidian blade begins life as a delicate sliver chipped from a lump of natural glass. at the moment of creation, an invisible clock is started. archaeologist ann freter. freter: when obsidian is first broken, it has a clean edge that absorbs hydrogen atoms from water at a known rate. it's the same kind of thing as if a piece of hard candy is left out. it will start to absorb a little bit of water and get softer and softer, while the center can still stay hard. obsidian is basically doing the same thing. and the longer that it's exposed to the elements, the thicker the hydration rim actually gets. and you can measure it and figure out a date. keach: the hydration layer is not visible to the naked eye,

so a tiny section of the obsidian blade is magnified. freter: what you're looking at is the hydration layer, which runs right across the very edge of the obsidian, where the hydrogen atoms have gone into the piece of obsidian. and it goes from this black line right here down to that black line right there, and it runs all the way across the entire piece of obsidian. and it's that line that you actually measure. keach: although the technique is simple, it's time-consuming. and right from the beginning, freter ran into problems with her dates. the copan valley was supposed to have been, basically, depopulated by around 850, 900 a.d. and some of our dates were as late as 1200 a.d., 300 years later than they should have been. and this was making absolutely no sense. and we were convinced the dating technique had a problem in it. keach: the prevailing theory of the maya collapse envisioned a slow population growth

until the fall of the dynasties about 850 a.d., when most of the maya were thought to have rapidly disappeared. but freter's obsidian dates suggested that people lived at copan for at least an additional 300 years and population declined gradually. either her dates or the prevailing theories were wrong. the contribution of a colleague would help resolve the puzzle. in 1984, david rue set out to study the impact of the maya on their landscape by collecting samples of ancient pollen from the trees and crops that once grew here. most plants reproduce by spreading their pollen on the winds. some of it has fallen in this bog.

as maya population grew, forests would have been cleared for cornfields. as population declined, trees would have grown up again. such changes in the ancient landscape should show up in rue's pollen samples. as we pull up this sample in this pollen coring tube that's about 7 meters below the current surface of this bog, we're essentially pulling up a sample of time, in which we've got a preserved record of vegetational changes through pollen that's been deposited in these sediments for thousands of years. keach: in his laboratory at penn state university, rue reconstructs the ancient changes in vegetation by counting individual grains of pollen from dated levels in the core. in levels dated to about a.d. 800, corn pollen appears in large quantities. maya farming was at its peak.

in later samples, he finds increasing quantities of pine pollen. pine trees would have been the first to reclaim abandoned cornfields. finally, he finds conclusive evidence of the end of maya farming in the valley at about a.d. 1200. rue: this is pollen from a tree in the mahogany family. this family is part of the plant community comprising the tropical deciduous forest, a very natural high-canopy rain forest type vegetation that would have occurred in the valley before the maya were there and then after they left the valley. based on our previous notions about the depopulation in copan, we would have expected this forest re-emerge by approximately 1000 a.d. or so. our pollen analysis showed that this forest was still completely gone from the valley up to and after 1200 a.d. -- a fact that indicated to us that people were still clearing the area for farms

for 200 years after we had expected the area to be abandoned. keach: as rue pondered the mystery revealed by his pollen samples, he consulted with colleague ann freter. and so we sort of commiserated with each other that this wasn't working, and what were we going to do, basically, and decided to sort of sit and think on it. a light bulb went off in our minds one day, and we realized, "hey, this is the way it really was !" people stayed here, and they continued to grow crops for at least another 200 years. we sat there and said, "what if it's right ?" if it's right, it means that the collapse was much more gradual. it means that a lot of our theories dealing with why the collapse occurred would have to be incorrect because they were based on a very sudden collapse as opposed to a gradual one. it's the two pieces of evidence meshing together, and each making much more sense than they would have individually.

the most interesting aspects of science are accidents. if you always get what you expected, then why are you doing it in the first place ? if you go out to prove a theory and your data supports that theory, that's fine. but if your data does not support the theory, you can't throw out the data. it's telling you your theory is wrong. and that's what happened in the case of the collapse at copan. keach: this is a representation of settlement in a.d. 500. using ann freter's dates, archaeologists could actually create a moving picture of the rise and fall of copan. as populations expanded, so did the agricultural fields. as they receded, the forests moved back into the valley. the evidence of a gradual process of population collapse seemed conclusive. a new theory would be required to explain it. initially, the theories were geared toward a very sudden catastrophic collapse,

such as a peasant revolt against the elite population or a major earthquake that made the area no longer habitable for a period of time or a major famine or epidemic that killed off masses of population. those have been theories that have been suggested for the maya collapse for hundreds of years. sanders: before we did the obsidian hydration project here at copan, most scholars thought that the maya collapse was a very rapid process. now, this was a pretty spectacular historical event, and it intrigued many scholars over the years. there were probably as many explanations as there were archaeologists. the most enduring explanation over the years, however, has always been an ecological one -- something must have happened to these people in terms of their relationship to their physical and biological environment. keach: could the collapse here have been caused by an ecological disaster ?

to support such a theory, sanders must first analyze the ability of the maya to harvest energy from their environment. sanders: the ancient maya, like most pre-industrial peoples, performed all the work using human muscular energy. and that energy was derived from maize, the staple crop. the construction of the great pyramids that you see and the temples and the carving of those stelae was all done with human labor. and so it's extremely important to know what the capabilities are of this valley in terms of supporting large populations, of sustaining a population for those kinds of activities. keach: today the copan valley supports about 25,000 people, almost as many as lived here just before the maya collapse. and these people are sustained by maize, or corn, as were the maya. by investigating how modern farmers earn a livelihood, bill sanders hopes to understand the capacity of the valley

to support ancient populations as well. he begins by talking to the farmers themselves. no hay frijoles sembrado en medio del maíz. no. este no hay. me dio diez cargas. y esas cargas de grano está hablando. en grano. en grano. okay. what he's telling me is that he's had this land nine years. and before that it was in cattle pasture. just like most of this land around here. and he got something like 1,200 kilos, and that yield is now only 1/3. this year he expects to get 1/3 of that. keach: it is not an isolated incident. by interviewing dozens of farmers, sanders learns that crop yields are declining throughout the valley.

this is because the fallow, or resting period, that soils need to regain their fertility has been cut short. this is exactly what sanders thinks may have happened in the past. sanders: when a subsistence farming population come into an area like this as pioneers, what they usually do is practice a long fallow system of farming. they'll crop land for a year or two, and then they'll let it go back to high forest again. then what happens, as population increases, you have to reduce the fallow because there just isn't enough land to let it all go back into high forest. and that process, at the end, can result in a landscape in which the land -- every piece of land is cultivated every year or every other year. and this is what's happened here. all the evidence that we have from the archaeological survey of the valley shows that at the peak of copan in the 8th century a.d., that the ratio of people to land must have been about as high as it is today in this area, possibly even higher.

and so the maya must have been cropping the land every other year or perhaps even every year, and that just cannot be sustained on a long-term basis. keach: as the ancient population grew, so did the demand for land. sanders and webster believe that maya farmers -- like these today -- were forced to cultivate even the steepest slopes, causing disastrous soil erosion. but could they find archaeological evidence to support their theory ? the results of overfarming the hillsides can be seen here in an excavation on the valley floor. while digging this mound in a modern cornfield, david webster discovs a ma dwelling buried by literally tons of topsoil. webster: we finally found the original floor down here which the maya built. and this wall sits right on top of it. we unexpectedly found this other little building here that they erected right next to it, creating this little corridor about 80 centimeters wide.

but what's really interesting are the deposits in this corridor. down here you've got about 15 or 20 centimeters of real garbage -- sherds and obsidian, the kind of material that pre-industrial people always discard near their houses. on top of it is about 70 centimeters of this eroded brown soil from the hillsides up to the north, which eventually covered this little platform. but we know the maya were still here because they placed this big slab partly on top of the little platform, but partly on top of the level of eroded soil that came across like this. right after that, the whole north wall of this big elite structure fell right down, creating all this collapsed debris, a couple of meters of it here, as you can see. but even that was eventually buried by even more erosion from those hillsides. so what we know is that while the maya were living here, they caused a lot of erosion. and after this building collapsed, it continued so that enough soil was coming off those hillsides up there to make it probably impossible for farmers

to get any kind of decent crop at all. keach: evidence from past and present converge to suggest that the maya collapsed because they could no longer grow enough food. to confirm this theory, the archaeologists would seek evidence of malnutrition in the ancient maya themselves. in 1989, stephen whittington is excavating a small settlement site a few miles from the copan acropolis. whittington: this site was occupied between about 650 and 900 a.d. in other words, it straddled the time of the peak and entered the first period of the collapse of copan. about two weeks ago, we put in a pit, and we immediately found a stairway made of very nicely cut stones. that indicates that somebody important lived on top, and that the structure itself had some important function for the entire site. one strange thing about the stairway

was that we discovered that three of the stones right in the center had moved out of place, for some reason, as if they had fallen into a hole. keach: the hole is a maya grave. the burial contains no jade or gold. but for the archaeologists, it holds a greater treasure. these are the remains of the actual people of copan. they reveal much about life in the ancient city. storey: skeletons are one of our best sources of information about the past that we can recover archaeologically. keach: archaeologist rebecca storey specializes in reading the bones for clues to ancient diet and health. this is a young maya woman. i know she's of high rank because she has the artifical deformation of the skull.

notice how it's flattened here in the front and in the back. her head was deliberately bound, probably by her parents, to give her this kind of a profile. besides the head flattening, though, she also has the nice dental mutilation -- this very nice elaborate kind of cross pattern -- done to her to, also, indicate her high rank. now, the importance of this skull is not so much the evidence of high rank but the evidence, instead, of stress. here on the back, i have the remains of what was iron deficiency anemia. the bone becomes very porous and spongy as the body reacts to the malnutrition in the diet. now, she was lucky. it healed. i have several skeletons at copan, however, that did not survive the episode. and here you can see the sponginess and porosity of the active lesion of the anemia. this individual, unfortunately, died before it could heal. keach: anemia can be seen in over 80% of storey's skeletons,

most of whom were maya nobles. commoners fared even worse. william sanders. all of the data from the copan project indicate that the best explanation for the maya collapse is that it was a human-induced ecological process. as a scientist, i'm never satisfied with the data that i have at hand. but all of the multiple lines of evidence are pointing in the same direction. the studies of modern agriculture, evidence of soil erosion from our archaeological excavations, the pollen analysis, the population history and, finally, the skeletal analysis all indicate that the maya were running into serious problems towards the end of the classic period. we think that the best explanation for the collapse here at copan is that there was a steady growth of population, increased intensification of agriculture. this caused disastrousffects on this landscape. and, also, the problems of adapting to that situation

caused severe nutritional stress and disease on the human population, all leading to a disappearance of the population by around 1200 a.d. keach: like all animals, we humans increase our numbers to fill the environments we inhabit. 800 years ago, the maya overreached their capacity to gain sustenance from this land, and they disappeared. a process that required more than 1,000 years in the past is now occurring in only a few generations. today, once again, about 25,000 people live in the copan valley. there is an ancient saying throughout most of latin america -- sembramos para cosechar. it means, literally, "we sow that we may reap." it is a metaphor that explains the dramatic population growth

common to most of the third world's farmlands. here the impulse to have children fits a traditional logic -- they provide the labor essential for survival in these rural villages. but these children, unlike their maya predecessors, are in no immediate danger of starvation. today food is imported into the valley. fertilizer nourishes fields that now are farmed without ceasing. and modern health care staves off disease... but only for a time. sanders: after the classic maya abandoned this valley around 1200 a.d., it wasn't reoccupied until early in the 19th century. since then, the population has steadily increased to reach a point today

approximately the same as it was just before the classic maya collapse. the difference is that in the 20th century, it has been increasing at a rate four times as fast. and what this means is that within 23 years, the population will be double what it was at the classic maya peak. webster: modern honduran farmers make short-term contingent decisions. they do what they have to do to keep themselves and their families going. now, the ancient maya went from one short-term decision to another and, ultimately, destroyed their basic resource, which was, of course, land. today we're doing what humans have always done. we're making immediate decisions without regard to their ultimate consequences. keach: mexico city is one of the largest and the fastest growing cities in the world. its population doubles every 30 years. the human avalanche comes from the countryside.

illegal settlements of squatters on the outside of the city are bursting at the seams. to discourage the growth of these illegal communities, the mexican government is encouraging settlement of the rain forest. if we could travel backward in time as easily as a helicopter carries us over the forests of mexico and guatemala today, we might capture a view much like this of ancient maya farmers. but these are modern pioneers. in a vast area of latin america, they are stripping away the rain forests -- thousands of acres every day. astonished american astronauts observed the result as they orbited high above the earth. this is a view of the yucatan peninsula. enlargement of one section of the photo provides startling evidence of mexico's resettlement policy. that sharp line is the border between mexico and guatemala.

it has been etched on the very surface of earth by colonists cutting down the rain forest to make agricultural fields. today, as in the past, advancements in technology provide a new level of living for some of us. but with this progress, the potential for destruction has been magnified 1,000 times. sanders: in the world today, we have two blocs of nations, both headed for an ecological disaster of worldwide proportions. on the one hand, we have western industrial civilizations with expanding economies, who monopolize the wealth of the world, who are polluting the air and water, possibly even destroying the atmosphere and consuming the very energy that sustains them. we have another bloc -- the agrarian nations, characterized by high fertility, but also, today, low mortality, which means that they are increasing at an unprecedented and exponential rate.

it is very doubtful that improvements in agricultural production can keep pace with that rate of growth. keach: of all the species on earth, only humankind can learn from the past. what lessons for the future will come from our voyage through time ? will our new perspective on planet earth convince us at last that all human societies and all human actions are forever intertwined ? the view from above is sobering. stains from topsoil bleeding from the hills of madagascar color the indian ocean. plumes of smoke billow from man-made fires in mozambique. perhaps it is but the scale of our impact on earth that has changed. perhaps we have yet to learn that out of the past

the mistakes of our ancestors do echo into the future. and if we listen closely, we may know they are speaking to us all. [ thunder ] captions by captionamerica, pittsburgh, pa.

funding for this program was provided by...

for announcer: the bare necessities of living healthy are easy. just eat right, be active, and have fun. yeah! go to mypyramid.gov to find out more.